Update doxygen and fix documentation (#5106)

Update Doxyfile to version 1.9.1

Re-enable doxygen CI checker

Fix some regular comments that should have been doc comments

Document void return types for some inline functions explicitly

Move start of some doxygen groups so they are included always, and not left
out of certain ifdefs

Ignore some doxygen warnings:
    Member (function) is not documented in headers
    Documented empty return type in headers
    Argument has multiple @param documentation sections

JerryScript-DCO-1.0-Signed-off-by: Máté Tokodi mate.tokodi@szteszoftver.hu

jerry-core/ecma/base/ecma-helpers-string.c

@@ -849,6 +849,8 @@ ecma_concat_ecma_strings (ecma_string_t *string1_p, /**< first ecma-string */
 
 /**
  * Increase reference counter of non-direct ecma-string.
+ *
+ * @return void
  */
 extern inline void JERRY_ATTR_ALWAYS_INLINE
 ecma_ref_ecma_string_non_direct (ecma_string_t *string_p) /**< string descriptor */
@@ -895,6 +897,8 @@ ecma_ref_ecma_string (ecma_string_t *string_p) /**< string descriptor */
 /**
  * Decrease reference counter and deallocate a non-direct ecma-string
  * if the counter becomes zero.
+ *
+ * @return void
  */
 extern inline void JERRY_ATTR_ALWAYS_INLINE
 ecma_deref_ecma_string_non_direct (ecma_string_t *string_p) /**< ecma-string */
@@ -1168,6 +1172,8 @@ ecma_string_copy_to_buffer (const ecma_string_t *string_p, /**< ecma-string desc
  * Convert ecma-string's contents to a cesu-8 string and put it to the buffer.
  * It is the caller's responsibility to make sure that the string fits in the buffer.
  * Check if the size of the string is equal with the size of the buffer.
+ *
+ * @return void
  */
 extern inline void JERRY_ATTR_ALWAYS_INLINE
 ecma_string_to_cesu8_bytes (const ecma_string_t *string_desc_p, /**< ecma-string descriptor */
@@ -2292,6 +2298,8 @@ ecma_string_trim_back (const lit_utf8_byte_t *start_p, /**< current string's sta
  * Used by:
  *        - ecma_string_trim
  *        - ecma_utf8_string_to_number
+ *
+ * @return void
  */
 extern inline void JERRY_ATTR_ALWAYS_INLINE
 ecma_string_trim_helper (const lit_utf8_byte_t **utf8_str_p, /**< [in, out] current string position */